1. Field of the Invention
The invention relates to durable cold-shrink articles formed from a highly flexible fluoroelastomer composition that resists attack and contamination by chemical and biological agents. More particularly the present invention provides fluorinated terpolymer compositions including monomers of tetrafluoroethylene, hexafluoropropylene and vinylidene difluoride. The fluorinated terpolymer compositions cure under selected conditions to provide a range of terpolymers varying in molecular weight. Multimodal mixed terpolymers, containing two or more terpolymers differing in molecular weight, may be formulated to satisfy cold-shrink product properties including, for example, tensile, elongation and permanent set.
2. Description of the Related Art
Elastomer materials have been developed for an impressive array of product applications. Of particular interest, for protecting and repairing signal and current carrying wires and cables, is a group of materials known generally as cold-shrink materials that provide highly elastic tubular structures. A conventional cold-shrink product typically comprises a flexible tube of an elastomer, such as EPDM rubber, held in expanded condition on a support core designed for removal from inside the flexible tube. The support core collapses on demand to allow the tube to shrink into contact with a wire or cable that needs protecting. Conventional cold-shrink products have limitations because inherent properties of currently used elastomers exclude them from use at high temperatures as barriers to aggressive, contaminating chemical and biological materials.
An answer to containment of biological and chemical hazards lies in the use of elastomers containing fluorine that belong to a group called fluoroelastomers. A number of investigators have confirmed that chemical resistance is a property of fluoroelastomers. For example, U.S. Pat. No. 4,690,994 describes a fluoroelastomer having advantages in common with conventional fluoroelastomers such as excellent heat resistance and chemical resistance. The same fluoroelastomer further shows excellent mechanical properties such as tensile strength and elongation, compression set, rebound resilience, processability, and resistance to the formation and growth of cracks.
U.S. Pat. No. 5,218,026, refers to a vulcanizable, fluorine containing elastomer composition which can be used to produce articles requiring high resistance to chemicals and solvents, such as fuel hoses, valves and O-rings, which are highly resistant to extraction when immersed in a fuel oil. This fluorine-containing elastomer composition has a molecular weight distribution exhibiting a plurality of peaks. According to U.S. Pat. No. 6,489,420, fluoroelastomers with high fluorine content have excellent permeation resistance to fuels. The fluoropolymer serves as a chemically resistant or vapor impermeable barrier.
In some cases the fluoroelastomer compositions have been manipulated to include a bimodal or multimodal distribution of molecular weights. U.S. Pat. No. 4,690,994 discusses a fluoroelastomer that has a bimodal molecular weight distribution of which the ratio (h2/h1) is in the range of 0.8 to 4.0. The ratio h2/h1, affects the balance of processability, mechanical properties, compression set and resistance to the formation and growth of cracks. An exemplary fluoroelastomer has a bimodal molecular weight distribution, which is composed of a higher molecular weight component and a lower molecular weight component. The weight average molecular weights (Mw) of these two components are in the range of 50-250×104 and 5-50×104 corresponding to the higher molecular weight component and lower molecular weight component respectively.
The value of multimodal molecular weight distribution is further described in U.S. Pat. No. 5,218,026 with regard to a fluorine-containing elastomer having a high molecular weight fraction that contributes to the mechanical properties of a molded article and a low molecular weight fraction contributing to the processability of the composition. In one embodiment, the content of fractions having a molecular weight not greater than 10,000 (M1) in the elastomer is preferably less than 15% by weight. The content of polymer fractions having a molecular weight of 2,000,000 or more (M200) in the elastomer is preferably in the range from 4-10% by weight.
U.S. Pat. No. 6,242,548 clarifies the term “multimodal terpolymer” to mean a terpolymer having two or more discrete molecular weight ranges. The multimodal terpolymer has a relatively low molecular weight component (A), a relatively high molecular weight component (B) and optionally an ultrahigh molecular weight component (C). Bimodal terpolymer compositions were shown to have values of elongation as high as 535%.
Even though fluoroelastomers have some properties of interest, further development is needed to confirm utility of such materials in cold-shrink applications particularly those requiring fluoroelastomers capable of maintaining an expanded condition over a significant period of time without failing by splitting, which renders them useless.